md/raid1: fix read balance when a drive is write-mostly.

When a drive is marked write-mostly it should only be the
target of reads if there is no other option.

This behaviour was broken by

commit 9dedf60313fa4dddfd5b9b226a0ef12a512bf9dc
    md/raid1: read balance chooses idlest disk for SSD

which causes a write-mostly device to be *preferred* is some cases.

Restore correct behaviour by checking and setting
best_dist_disk and best_pending_disk rather than best_disk.

We only need to test one of these as they are both changed
from -1 or >=0 at the same time.

As we leave min_pending and best_dist unchanged, any non-write-mostly
device will appear better than the write-mostly device.

Reported-by: Tomáš Hodek <tomas.hodek@volny.cz>
Reported-by: Dark Penguin <darkpenguin@yandex.ru>
Signed-off-by: NeilBrown <neilb@suse.de>
Link: http://marc.info/?l=linux-raid&m=135982797322422
Fixes: 9dedf60313fa4dddfd5b9b226a0ef12a512bf9dc
Cc: stable@vger.kernel.org (3.6+)

1 files changed, 3 insertions, 2 deletions

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 4153da5d4011..d34e238afa54 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -560,7 +560,7 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
                 if (test_bit(WriteMostly, &rdev->flags)) {
                         /* Don't balance among write-mostly, just
                          * use the first as a last resort */
-                        if (best_disk < 0) {
+                        if (best_dist_disk < 0) {
                                 if (is_badblock(rdev, this_sector, sectors,
                                                 &first_bad, &bad_sectors)) {
                                         if (first_bad < this_sector)
@@ -569,7 +569,8 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
                                         best_good_sectors = first_bad - this_sector;
                                 } else
                                         best_good_sectors = sectors;
-                                best_disk = disk;
+                                best_dist_disk = disk;
+                                best_pending_disk = disk;
                         }
                         continue;
                 }